Organic electroluminescent elements (hereinafter, an organic electroluminescent may be referred to as “organic EL”) are light-emitting elements in a structure having a light-emitting layer containing a light-emitting compound held between negative and positive electrodes, in which excitons are generated by recombination of electrons and positive holes injected into the light-emitting layer and light (fluorescence or phosphorescence) is emitted when the excitons are inactivated. These elements are attracting attention recently, as they emit light even at a voltage approximately of several to several tens V, and are characteristic of wide view angle and high visibility because they are self-emitting type elements and also superior in size and portability because they are thin film-shaped completely solid-state elements. The organic EL panels, which have such an organic EL element formed on a substrate, can be used as display devices, displays, and various emission light sources.
Currently, particularly among the organic EL panels, flexible displays, which can be used as folded, are attracting attention. For production of flexible displays, a flexible material should be used as the substrate for such an organic EL element. In addition, organic solids used in organic EL elements, such as light-emitting layer (organic emitter), electron, and positive hole transport materials, had a problem that they are generally extremely unstable to water and oxygen, and degraded by water and oxygen present in organic EL element and also by water and oxygen penetrating into the element from outside, leading to growth of so-called dark spots, deterioration in light permeability and thus drastic decrease in luminous efficiency. For that reason, the substrate should have high gas barrier properties. Use of an ultrathin glass plate as the substrate for that reason is known to satisfy the requirements both in flexibility and gas barrier properties (reference 1).
However, the ultrathin glass plates are very brittle and, if an ultrathin glass plate is conveyed alone in the production process for forming an organic EL element on the ultrathin glass plate, there is a problem of decrease in yield caused by generation of “fractures” and “cuts” in the ultrathin glass plate during conveyance. Known as the methods of preventing the “fractures” and “cuts” during conveyance are a jig-set method of conveying the ultrathin glass plate, as it is temporarily fixed with a jig, a sheet-bonding method of conveying the ultrathin glass plate, as it is temporarily bonded to an adhesive sheet, and a substrate-bonding method of conveying the ultrathin glass plate, as it is bonded to a support with a wax or bonding agent. However, in the case of the jig-set method, a jig suitable for each treatment process should be selected and used, as residual of the chemical solution and unevenness of the treatment during WET treatment (chemical solution treatment) are taken into consideration. Accordingly, the method has a problem of low maintenance efficiency and low processability. Alternatively in the case of the sheet-bonding method, the ultrathin glass plate may be fractured by the stress applied for separation of the adhesive sheet that became unneeded after production process. Alternatively in the case of the substrate-bonding method of conveying the ultrathin glass plate, as it is bonded to a substrate with a wax or bonding agent, an additional step of removing the wax or bonding agent deposited on the rear face of the ultrathin glass plate with organic solvent should be installed after production process, which makes it difficult to improve productivity.
Japanese Unexamined Patent Publication No. 2003-292916 (Patent Document 2) describes an thermal release adhesive sheet (heat-peelable adhesive sheet) that is an adhesive sheet used for temporary fixing of a brittle bonding target, characterized in that even a brittle bonding target can be separated and recovered easily without damage by heat treatment and at least one face of the base substance has a heat-expanding adhesive layer containing heat-expandable microspheres. The heat-expandable microsphere contains hydrocarbon gas in a spherical rubber-like elastic body and has a property that the rubber-like elastic body expands and/or foams, as the hydrocarbon gas expands when heated at a temperature of more than a particular temperature. However, although there is no thermal expansion when heated at a temperature not higher than the expansion and/or foaming temperature, the hydrocarbon gas may be released externally as offgas at a temperature close to it. If the thermal release adhesive sheet is used as a sheet for temporary fixation of the ultrathin glass plate, when an organic electroluminescent element is formed on an ultrathin glass plate by vacuum deposition method in the production step for organic EL panels, hydrocarbon gas may be generated in a great amount in the vacuum chamber, causing decrease in the degree of vacuum and elongation of the period needed for the vacuum chamber to reach a desired degree of vacuum, causing a problem of drastic deterioration in film precision and productivity. In other words, there is currently no available process for producing an organic EL panel superior in flexibility and gas barrier properties by using an ultrathin glass plate, by which it is possible to protect the ultrathin glass plate from generation of “fractures” and “cuts” during conveyance in the production process, to produce the organic EL element efficiently by evacuating the system to a particular degree of vacuum rapidly when an organic EL element is formed by vacuum deposition, and to recover the organic EL panel obtained without damage of the ultrathin glass plate after production process, and in which there is no need for an additional cleaning step of removing the adhesive substance and others deposited on the rear face of the ultrathin glass plate.